1. Field of the Invention
The present invention relates to an image forming apparatus that uses a developing unit having a structure in which developer, conveyed along a certain circulation channel, is held on a moving surface of a developer holding body to contribute to a development process, and then returned from the surface of the developer holding body to the circulation channel.
2. Description of the Related Art
Japanese Patent Application Laid-open No. H9-160364 discloses a developing apparatus that is a known example of this kind of developing unit. This developing apparatus is shown in FIG. 1. In FIG. 1, a developing apparatus 900 includes a circulation channel for conveying developer, not shown, including toner and magnetic carrier in a circulating manner in a casing, and a developing roller 910. The circulation channel further includes a first developer container 901 and a second developer container 903 that are arranged in the short-length direction with respect to each other. The developer contained in the first developer container 901 that is a part of the circulation channel is conveyed along the longitudinal direction of the space in the first developer container 901, that is, in the direction shown by an arrow A in FIG. 1, by way of driving rotation of a first conveyor screw 902. The first developer container 901 and the second developer container 903 positioned adjacent thereto are communicatively connected at each longitudinal end thereof. The developer that is conveyed toward the end of the arrow A in FIG. 1 in the first developer container 901 by way of driving rotation of the first conveyor screw 902 goes through the connected portion, and enters the second developer container 903. The developer is then conveyed to the direction shown by an arrow B in FIG. 1 that is the direction opposite to that shown by the arrow A by way of driving rotation of a second conveyor screw 904 in the second developer container 903. When the developer is conveyed to the end of the arrow B in the second developer container 903, the developer goes through the connected portion, and enters the most upstream area of the first developer container 901 along the direction shown by the arrow A. In this manner, the developer is conveyed in a circulating manner in the first developer container 901 and the second developer container 903.
The developing roller 910 is arranged at the short-length lateral side of the second developer container 903. The developing roller 910 includes a developing sleeve having a non-magnetic pipe that is driven to rotate, and a magnet roller, not shown, that is held inside the developing sleeve in an unrotatable manner. The developer in the second developer container 903 is carried on the surface of the rotating developing sleeve by way of the magnetic force from the magnet roller, and conveyed to a developing area where the developing sleeve faces a photoconductor not shown. After the surface of the sleeve is developed with the developer, the developer on the surface of the sleeve is collected to the second developer container 903. The concentration of the toner in the developer becomes reduced after the toner contributed to the development process. A controlling unit not shown calculates the number of pixels in an image based on image information thereof, predicts how much toner is consumed for developing the image based on the calculation result, and drives a toner supplying unit, not shown, for a length of time corresponding thereto. Toner is then added to the developer in the first developer container 901 through a toner supplying opening 915 provided near the end of the first developer container 901 at the upstream of the conveyed developer. In this manner, the toner concentration of the developer is recovered. According to such a toner supplying structure, the toner concentration can be recovered more quickly in comparison to a structure where the toner is added after a toner concentration sensor detects a decrease in toner concentration.
However, in the development process, the toner is not necessarily consumed in the amount that is predicted based on the number of pixels in the image. It is possible for the amount of toner consumption to fall below or exceed the prediction, depending on factors such as environmental variation. If the toner is kept being added based on the prediction having such an error with respect to the actual toner consumption, the toner concentration in the developer cannot be maintained appropriately.